The purpose of this project is to study the physical, chemical, and ultrastructural properties of these salts, and to clarify the kinetic and thermodynamic processes and the interactions with substances of biological interest that uniquely enable calcium phosphate salts to carry out their specialized role in vivo. The properties of calcium phosphate salts are being studied with a variety of ultrastructural and physical-chemical techniques such as electron microscopy, x-ray diffraction, surface area analyses, chromatographic and standard analytical chemistry procedures. Two principal endeavors are currently being pursued. In one, artificial lipid vesicles (liposomes) are used as in vitro models to investigate physico-chemical aspects of matrix vesicle (MV)-mediated calcification in vivo. Present findings show that enzymatic breakdown of extraliposomal proteoglycans (PG) does not destroy the retarding effect PGs have on calcium phosphate precipitation in liposomal suspensions. Core protein as well as glycosaminoglycan components, but not hyaluronic acid, are equally as effective as intact PG in delaying precipitate development. Other liposome studies show that of the MV lipid membrane constituents, phosphatidylserine and sphingomyelin may have the greatest controlling influence on expansion of intravesicular crystals into the surrounding matrix. In the second major endeavor, the hydrolytic conversion of anhydrous dicalcium phosphate to apatite is being investigated as a reaction model for studying the growth of apatite crystals under controlled solution conditions. Findings to date indicate that the Gibbs free energy of precipitation is a major factor in determining the final size attained by the apatite crystals.